The invention arises from the object of developing a method by which the turbine of a gas turbine power station can not only be operated as a conventional turbine with a directly coupled compressor group but also as a pure storage air gas turbine which draws its combustion air exclusively from an air reservoir, and can be fed in any given mixture ratio from these two compressed air sources.
In order to satisfy these requirements, a conventional gas turbine installation must be designed for a compressor pressure ratio which makes it compatible for operation with an air reservoir, generally speaking therefore with an underground air storage cavern. This means that if, for example, the pressure in the cavern is 50 bar at approximately ambient temperature, the pressure ratio of the compressor group should be .sup..pi. 50. Stated more precisely, the compressor final pressure must be at least equal to the pressure in the storage cavern during types of operation in which the compressor group supplies air, either exclusively or in part, into the storage cavern.
In present day air storage gas turbine installations, the pressure ratio is in the range between 40 and 70. It is produced by multistage industrial axial compressors with multiple intercooling and after-cooling of the compressed air before it enters the storage cavern. The output of these compressors, in known air storage gas turbine power stations, is substantially less than the air mass flow necessary for the full power of the turbine, for example a quarter of it. This means that, during the low load periods in which the compressor group is driven by the generator--used as a motor,--the compressor group must operate four times as long in order to replace the air used by the turbine from the storage cavern. Under certain circumstances, this can mean an intolerable limitation to the available electrical operating capacity, in particular to the time of operation of the turbine. However, even if the compressor mass flow is selected to be the same as or even greater than the turbine mass flow, only the air quantity supplied by the compressors to the cavern can serve, as a maximum, as operating air for the turbine. As soon as this is consumed, it is necessary to change back from power operation to pumping operation. It is, therefore, only possible to store and subsequently generate electrical energy during time intervals limited to a greater or lesser extent.
The relationships are different in the case of ordinary gas turbine power stations. In these, the compressor is permanently driven by the turbine and continuously and instantaneously provides the combustion air needed by the turbine so that any given length of operating period is possible. Such gas turbine installations, however, are pure thermal electricity generators and do not permit storage of electrical energy. At times of low electricity consumption (i.e., in part load operation), their efficiency decreases substantially so that part load operation can become economically unfavourable.